Problem: If the least common multiple of two 6-digit integers has 10 digits, then their greatest common divisor has at most how many digits?
Explanation: Call the two integers $a$ and $b$. Recall that the product of two numbers' LCM and GCD is equal to the product of the two numbers themselves: $$\mathop{\text{lcm}}[a,b]\cdot \gcd(a,b) = ab.$$This can be rearranged to give $$\gcd(a,b) = \frac{ab}{\mathop{\text{lcm}}[a,b]}.$$In this case, we know that $a<10^6$ and $b<10^6$, so $ab<10^{12}$. We also know that $\mathop{\text{lcm}}[a,b]\ge 10^9$, since the smallest 10-digit number is $10^9$.

Therefore, $$\gcd(a,b) < \frac{10^{12}}{10^9} = 10^3,$$so $\gcd(a,b)$ has at most $\boxed{3}$ digits.

(We should check that there are actual integers $a$ and $b$ for which $\gcd(a,b)$ has $3$ digits. There are; for example, we can take $a=500{,}000$ and $b=200{,}100$, in which case the least common multiple is $1{,}000{,}500{,}000$ and the greatest common divisor is $100$.)